The present invention relates to a process for the removal of water from solutions containing formaldehyde for the production of formaldehyde.
Formaldehyde is a commodity chemical that is conveniently produced and transported in the form of formalin solutions. Formalin solutions contain typically between 30% and 60% formaldehyde, the balance of the solution being mostly water, usually with some methanol present. The formaldehyde is predominantly present as complexes with water or methanol in the form of glycols of hemiformals. There are a number of methods described in the patent literature for the dehydration of formaldehyde solutions with the object of producing a dry monomeric formaldehyde stream. For example U.S. Pat. No. 4,962,235 describes the purification of a formaldehyde/water/methanol mixture by distilling in the presence of a polyalkylene oxide such that formaldehyde vapour is produced at the top of the column and polyalkylene oxide, water and methanol are removed from the bottom of the column.
NL-A-6814946 describes a process for recovering formaldehyde from aqueous formaldehyde streams by contacting the stream with C6-C10 aliphatic alcohol which reacts with the formaldehyde to form a hemiformal which can be separated from the water and subsequently dissociated back to the alcohol and formaldehyde components.
U.S. Pat. No. 3,174,912 describes a process for the removal of water and coloured organic impurities from a dilute aqueous formaldehyde mixture by distillation in the presence of acetone. A mixed stream containing acetone and formaldehyde and a small amount of water is removed from the top of the column and separated by partial condensation into a stream containing a relatively small amount of formaldehyde in acetone and a stream containing a larger proportion of formaldehyde in acetone.
It is, however, desirable to avoid the introduction of additional chemical compounds into a process which requires purified formaldehyde because the additional compound may need to be removed from the process at a subsequent stage.
One process in which formaldehyde is used is that for the production of methyl methacrylate by the reaction of methyl propionate with formaldehyde in the presence of methanol. The use of formaldehyde as a readily available formalin solution introduces water into the reaction which may have a deleterious effect on the catalyst used and is likely to promote the hydrolysis reactions of the methyl propionate reactant and of the methyl methacrylate product. Water is produced as a by-product of the methacrylate synthesis reaction and it is therefore desirable to reduce to a minimum the amount of water that is introduced into the reaction zone with the feeds so that the level of water in the reactor is maintained as low as possible. Processes for the production of methyl methacrylate, in which propionic acid or its methyl ester is reacted with formaldehyde or methylal in the presence of methanol, are disclosed in U.S. Pat. No. 3,535,371, U.S. Pat. No. 4,336,403, GB-A-1107234, JP-A-63002951. However there is no disclosure in these references of how to prepare the feed materials, particularly the formaldehyde, for the required reaction.
U.S. Pat. No. 4,040,913 discloses the use of water entraining compounds such as benzene, toluene or methylisobutyl ketone, and FR-A-2409 975 discloses the use of saturated C4-C6 hydrocarbons. GB-A-1301533 discloses the use of alkanols being C3-C6 compounds but the alkanols are not used as water entraining compounds.
It is therefore an object of the invention to provide a process for the preparation of a formaldehyde feed product from a formaldehyde-containing solution.
It is a further object of the invention to provide a process for treatment of a formaldehyde-containing solution to yield a formaldehyde feed suitable for use in a methyl-methacrylate production process which contains a reduced amount of water compared to the original formaldehyde-containing solution.
According to the invention, a process for separating a formaldehyde-containing product from a formaldehyde solution comprising formaldehyde, water and methanol, wherein said formaldehyde-containing product contains substantially less water than said formaldehyde solution, comprising distilling said formaldehyde solution in the presence of a water entraining compound.
The water entraining compound is selected such that it is capable of dissolving water, formaldehyde, methanol and hemiformal compounds formed by the reaction of formaldehyde with methanol. The water entraining compound is preferably a saturated or unsaturated carboxylic acid or ester or a carbonyl compound which is substantially unreactive towards formaldehyde under the conditions of the distillation and also able to entrain the water, preferably by forming a minimum boiling azeotropic mixture with water Preferably the water entraining compound forms a heterogeneous minimum boiling azeotrope with water. Suitable compounds include C4-C8 alkanoic acids and their lower alkyl, e.g. C1-C8, esters, and ketones having at least 4 carbon atoms such as diethyl ketone. Particularly preferred compounds are esters and methyl propionate and methyl methacrylate have been found to be especially useful in certain processes. It is greatly preferred to use a compound which is intended to be introduced into a process in which the dewatered formaldehyde product is intended to be used.
One particularly preferred compound for use in dewatering a formalin solution to produce formaldehyde suitable for use in a process to react formaldehyde with methyl propionate in the presence of methanol is methyl propionate. In one preferred form of the invention we therefore provide a process for separating a formaldehyde-containing product from a formaldehyde solution comprising formaldehyde, water and optionally methanol, wherein said formaldehyde-containing product contains substantially less water than said formalin solution, comprises distilling said formaldehyde solution in the presence of methyl propionate.
According to a second aspect of the invention we provide a process for the production of methyl methacrylate by the reaction of formaldehyde with methyl propionate in the presence of methanol and of a suitable catalyst, wherein said formaldehyde is produced from a formaldehyde solution by means of distilling said formaldehyde solution in the presence of methyl propionate. Suitable catalysts are known in the art and include silica catalysts having alkali metal sites.
The process of this aspect of the invention is beneficial, in that formaldehyde feed is recovered as a complex with methanol in a methyl propionate rich stream and water is removed from the process substantially free of organics. Integration of the process for producing formaldehyde of the present invention with a methyl methacrylate producing process as described has the additional advantage that the overall energy requirement for the combined process may be reduced compared with other formaldehyde dewatering methods.
Although we have found that the process of the present invention is particularly suitable to provide a formaldehyde feedstock for subsequent reaction for methyl methacrylate production, the invention is not limited to the production of formaldehyde for such a process and may be suitable to produce dewatered formaldehyde for other applications.
The formaldehyde solution is preferably standard formalin which usually contains water and formaldehyde in approximately equal proportions, usually with a small quantity of methanol. At least some of the formaldehyde is normally present as various water-formaldehyde or methanol-formaldehyde adducts. Normally the term xe2x80x9cformaldehydexe2x80x9d will be used here to refer to the total formaldehyde, whether present as free formaldehyde or in the form of such adducts. The composition of formalin solutions may vary and the process of the invention may be operated for a variety of different formalin compositions.
It is preferred that the formaldehyde solution is pre-mixed with methanol before the distillation process. The methanol/formalin mixture is preferably allowed to equilibrate so that formation of methanol-formaldehyde adduct species is promoted, e.g., by allowing the mixture sufficient time to equilibrate, or by stirring or adjusting the temperature of the mixture. Preferably a suitable amount of methanol is used to provide a molar ratio of methanol to formaldehyde of 0.3-1.5:1, more preferably 0.5-1.2:1, especially 0.8-1.1:1. The methanol may be provided in the form of a mixture with methyl propionate, for example to enable recirculated methanol from the distillation or form an associated process or process step to be used.
The amount of water entraining compound introduced into the distillation is in excess of the amount required to form an azeotropic mixture with the water and preferably also with any methanol present so that the water in the mixture is more volatile than the formaldehyde adducts. Methyl propionate forms an azeotropic mixture with water comprising 92% methyl propionate and 8% water. The methyl methacrylate/water azeotrope contains about 14% water (by mass) and the diethyl ketone/water azeotrope contains about 84% diethyl ketone by mass. Preferably the relative proportion of water entraining compound to formaldehyde in the base of the column is in the range 5:1-20:1, e.g. about 10:1 by mass. However, when the water-entraining compound is refluxed, the amount fed to the column may be adjusted as necessary.
The majority of the water is removed as a mixture with the water entraining compound. A stream containing most of the water may be conveniently removed from the distillation process as a liquid sidestream, e.g. by using a chimney tray or similar device at an appropriate position in the column. A suitable position for withdrawing such a sidestream may be determined by considering the composition of the liquid phase throughout the column by known methods in the art. Methyl propionate and water form a heterogeneous azeotrope comprising 92% methyl propionate w/w. This azeotropic mixture may be separated in a decanter and the organic phase thus produced, which comprises mainly methyl propionate may be refluxed into the distillation process. The aqueous phase produced from the decanter may be further treated in a second distillation unit, preferably at elevated pressure, to give an aqueous stream which is largely free of methanol, formaldehyde and methyl propionate. The overheads from the second distillation unit can be recycled back to the main column or to the decanter or to the formaldehyde solution pre-treatment, if present.
Most of the formaldehyde is taken as a bottom product as a mixture with the water entraining compound. This mixture may be used directly if required for a further reaction in which both compounds are used together, e.g. in the synthesis of methyl methacrylate. This formaldehyde-containing mixture contains substantially less water than the formaldehyde solution fed to the process. For example, in a typical process according to the invention using methyl propionate as the water entrainer, a formalin solution containing formaldehyde and water at a weight ratio of approximately 1:1 may produce a dewatered formaldehyde stream containing formaldehyde and water at a weight ratio of about 10:1.
The process of the present invention is preferably carried out in such a way that the purified formaldehyde product may be used in a further process for which it is required. Preferably the process of the present invention is integrated with such a further process so that the purified formaldehyde product is supplied directly to that further process. When the formaldehyde product from the process of the present invention is intended to be used as a feedstock together with the water entraining compound in a further process, e.g. for the production of methyl methacrylate from formaldehyde and methyl propionate when methyl propionate is used as the water-entraining compound, then the process of the present invention and the manufacture of methyl propionate may beneficially be operated in proximity to or integrated with the further process.
In a preferred arrangement, methyl propionate is produced by a process which provides a source of methyl propionate mixed with methanol and, optionally, water. This mixture may be fed to the distillation of the present invention to extract the formaldehyde from a formalin solution. In this way, the separation of the methyl propionate from the process in which it is produced may be avoided. When the feedstocks used contain methanol as described, an azeotropic mixture of methyl propionate and the methanol not complexed with the formaldehyde may be removed from the top of the distillation column. That mixture may be recycled or removed to storage or a further process.
When the formaldehyde from the process of the invention is intended to be reacted with methyl propionate and methanol to produce methyl methacrylate, then the process of the present invention is especially convenient, particularly if used in conjunction with a methyl propionate-producing process as described above. Thus, in a second preferred arrangement, the dewatered formaldehyde mixture with methyl propionate is used, directly or after an intermediate treatment, in such a methyl methacrylate process. Similarly, a methyl propionatexe2x80x94methanol stream, which may be derived from a process to produce methyl propionate, and used in the present invention may also be used to make methyl methacrylate in an integrated process.